It often is desirable for motor vehicles to have retractable window shades. If there is sufficient space for window shades in motor vehicles, the window shade can be pre-assembled as a component complete with housing or installation frame. The entire unit is installed as a whole at the assembly line.
Such assembly procedures prevail, for example, in connection with rear window shades. However, conditions are different if the window shade is to be housed in doors or behind the side paneling of motor vehicles. Often, space at such locations is not sufficiently large to enable installation of a completely pre-assembled unit. In such case it is necessary to fasten bearing devices for the windup shaft directly on appropriate elements on the motor vehicle body and then later insert the windup shaft with the window shade fastened thereon. However, a difficulty arises in that the window shade, even if it is electrically operated, is customarily retracted with the aid of a spring drive housed in the windup shaft. In order for the spring drive to properly perform its function, it must be appropriately wound so that sufficient winding force is available, even when the window shade web is in a completely wound up condition.
Properly winding of the spring drive at the assembly line also is difficult, particularly because an exact predetermined number of turns for winding is required. If the pre-tension is set too high, the spring drive reaches the limit before the window shade web is completely extended and blocks further extension. But if the spring is insufficiently wound, pre-tension of the window shade web when wound up is insufficient to assure that it is housed without rattling.